Many of the commercial pipelines in industries are insulated and operate at high temperatures. Currently, these pipes are inspected only during annual maintenance shutdown by stripping of the insulation. The insulation is then replaced after inspection at considerable costs. Inspection of pipes for flaw detection at high temperature has been a critical issue in many industries, especially nuclear and oil industries where shutdown of plants may incur heavy losses. Moreover, thermal shocks caused due to the shutdown may result in weakening of the pipe material and hence to avoid this slow cooling and heating rates are maintained, this slow cooling and heating may sometime take days.
Piping systems are often inspected ultrasonically to ensure safety. This can be accomplished by a series of point test but from outside the pipe. If insulation covers the pipe, as is often the case, access to pipe requires removal of the insulation to perform the test and then reinstallation when the test is complete. Removal and reinstallation of coating is not only time consuming but in most cases expensive too. There is therefore an urgent need for development of a quick, reliable method for the detection of cracks and corrosion under insulation. Long-range, longitudinal and torsional guided wave generated in pipes using Magnetostrictive sensors (MsS) have great potential for application to structural health monitoring of hard to inspect pipes.
The use of Guided waves had been an area of interest to many scientists particularly because of its immense capability in Long Range Ultrasonic Techniques (LRUT). This technique besides being cost effective is also very simplistic and user friendly. Guided waves usually refer to mechanical waves in ultrasonic frequencies that propagate in bounded medium (usually pipes and rods), these waves are confined within the geometries and are guided by geometric boundaries, and hence it is called as guided waves.
Generation of guided waves has taken place using piezoelectric principles or Magnetostrictive technology. Magnetostrictive Sensors (MsS) are used widely to monitor pipelines in industries. But, still the use of MsS at high temperatures remains a challenge. This is mainly because of the usual configuration used in the design of sensors.
A typical MsS Technique employs the configuration as shown in FIG. 1. [1] MsS uses a “permanent magnet” to obtain the bias field and “adhesives” to bond the magnetostrictive tapes. These permanent magnets easily get demagnetized at very high temperatures which cause a loss in signal strength. The sandwiched adhesives interfaced between the pipe and magnetostrictive tape also get removed at high temperatures causing the delamination of sensors; thereby causing a large noise in the signals due to air coupling. The permanent magnets was considered essential in prior art because of the high value of bias required in the currently used Fe—Co strips in MsS Sensors.
The basic principle behind obtaining guided wave using MsS is the phenomenon called magnetostriction. The magnetization forces and magnetostrictive forces both operate to give rise to guided wave propagation. The latter one of these two is the usual force only in ferromagnetic materials. Since, all our study for this invention is mainly on mild steel pipes which are ferromagnetic, though the invention is not limited to this;
Magnetization Force Mechanism: Magnetization forces occur only in ferromagnetic Materials. The grains present in the material act as magnetic dipoles. In the presence of a biasing magnetic field, these dipoles tend to align in a direction and create magnetization inside the material. When an oscillating magnetic field is applied through the excitation coil, these magnetic dipoles experience force. This oscillating body force results in the propagation of acoustic waves inside the material. An equation given by Thompson [3] for this force isfM=μ(M0·∇)H 
Here, fM=Force due to magnetization.
MO=Magnetization vector of ferromagnetic material.
H=Magnetic field.
μ=Magnetic Permeability.
Magnetostriction Force Mechanism: A normalized dimensional change due to application of external magnetic field depending on the direction and magnitude of the field is called magnetostriction. It originates from the very fact that all main interactions between the atomic magnetic moments in solids depend on the distance between them (e.g. exchange interaction, dipole-dipole interaction, interaction of magnetic moments with crystal electric field). Here we will deal with only two kinds of magnetostriction effects. One is Joules effect used in transmission of waves and the other is Villari effect used in receiving of signals.
Thus, the main object and other objects of the invention is to obtain a good magnetization and magnetostrictive force from a material which has a capability to align its dipoles at very low bias field, or a material that has a very steep linear region in magnetostrictive curve and at the same time should possess a good magnetostriction constant.